1. Field of the Invention
The present invention relates to a semiconductor device having a driver section for transmitting data to a data transmission line and a driver control method. In particular, the invention relates to improving a transmission characteristic by providing a driver with a linear current-voltage (I-V) characteristic in the interface having power supply termination (method of pulling up a power supply with a resistor at the end of a transmission line). Furthermore, the invention relates to a semiconductor device which can allow a driver with a linear I-V characteristic to be adapted to various conditions, and a driver section control method.
Priority is claimed on Japanese Patent Application No. 2007-271144, filed Oct. 18, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
In recent years, there has been a case in which power supply termination of a transmission line is performed to prevent deterioration of the waveform quality caused by reflection as the operation speed increases. As shown in FIG. 6, in the case of transmitting data from a side of a driver 21 to a side of a receiver 22 through a data transmission line 23, Vtt termination is performed by a termination voltage Vtt and a termination resistor Rtt at the side of the receiver 22.
FIGS. 7A and 7B show Eye patterns (eye diagrams) of simulation waveforms in the data transmission line 23. FIG. 7A shows an Eye pattern when a voltage level of the termination voltage Vtt is set to ½ of a power supply voltage VDD. FIG. 7B shows an Eye pattern when the voltage level of the termination voltage Vtt is set to the power supply voltage VDD. Usually, a reference voltage VRef used to determine a low level/high level of a signal is set to a level of ½ of the power supply voltage VDD in such a manner that a ‘margin between the low level and the reference voltage VRef’ becomes equal to a ‘margin between the high level and the reference voltage VRef’.
The signal amplitude at the time of Vtt termination depends on a resistance value of the termination resistor Rtt, and the signal amplitude increases as the resistance value increases. When the Vtt termination is not performed, a signal is fully amplified. As indicated by a simulation result of FIG. 7A, in the case when Vtt termination is performed, it is necessary to make a signal cross point a1 of the Eye pattern equal to the level of the reference voltage VRef. Therefore, it is necessary to make the termination voltage Vtt equal to a level of ½ of the power supply voltage VDD, that is, the level of the reference voltage VRef.
However, a demand for low power consumption in recent portable devices represented as mobile phones has increased. Also in a semiconductor device, it is necessary to reduce a power supply voltage in order to realize low power consumption. In addition, from a point of view of an improvement in signal processing speed and the like, a demand for an increase in signal speed has also increased. For this reason, it is necessary to realize both the low voltage and the high speed.
As the power supply voltage decreases, the reference voltage VRef also decreases. Taking the sensitivity, operation speed, and the like of a receiver into consideration, a possibility that the low voltage and the high speed will be realized increases if a level of the reference voltage VRef can be made higher than a level of ½ of the power supply voltage VDD. Under such a background, when the Vtt level of Vtt termination is made equal to the power supply voltage, the Eye pattern shifts to a side of the power supply voltage VDD (upper side in the drawing) as indicated by the simulation result of FIG. 7B. As a result, the level of the reference voltage VRef can be set higher than the level of ½ of the power supply voltage VDD.
The simulation results shown in FIGS. 7A and 7B are obtained by modeling a driver section model as ON resistance of a transistor. Accordingly, those simulation results are results when a relationship (I-V characteristic) between a current and a voltage on the pull-up/pull-down side of the driver section has a linear characteristic. However, since an I-V characteristic of an actual transistor shows a characteristic having a non-saturation region and a saturation region, the I-V characteristic does not always have a linear characteristic. These characteristics are shown in FIG. 8. It is revealed that a pull-up current and a pull-down current increase with decreasing the ON resistance of the transistor (with increasing a gate width of the transistor) while the pull-up current and the pull-down current decrease with increasing the ON resistance of the transistor (with decreasing the gate width of the transistor).
As is apparent from the simulation results, a steady state of a high level is set to a level equal to the power supply voltage when the Vtt level is made equal to the power supply voltage VDD. It is necessary to decide a resistance value of the termination resistor Rtt and an ON resistance value of a driver transistor in such a manner that the ‘margin between the low level and the reference voltage VRef’ becomes equal to the ‘margin between the high level and the reference voltage VRef’.
Here, a ‘transistor model’ in which the ON resistance of a driver transistor is added at a level (FIG. 8; a point a2 shown by O) of ½ of the power supply voltage VDD is considered. This is set such that a voltage-divided level by the ON resistance on the pull-up side and the pull-down side becomes a ½ VDD level. Using the ‘transistor model’ and a ‘resistance element model’ modeled as the ON resistance, simulation results when the Vtt level is made equal to the power supply voltage VDD are shown in FIG. 9.
FIG. 9A shows an Eye pattern when the ‘resistance element model’ is used. FIG. 9B shows an Eye pattern of the ‘transistor model’. In the Eye pattern of the ‘resistance element model’ shown in FIG. 9A, there is a symmetric property in a signal. In the ‘transistor model’ shown in FIG. 9B, however, there is no symmetric property in the Eye pattern (jitter occurs) and a fluctuation of a waveform in a LOW level is noticeable.
In the simulation results shown in FIG. 9, it can be seen that the stability in a steady state of the low level is reduced in a driver section with a nonlinear I-V characteristic.
In this case, the low level in a steady state can be appropriately set by making a resistance value of the termination resistor Rtt small. The simulation results are shown in FIGS. 10A and 10B. FIG. 10A shows an Eye pattern when the ‘resistance element model’ is used. FIG. 10B shows an Eye pattern when the transistor model’ is used.
The simulation result of the transistor model shown in FIG. 10B clearly indicates the following points. Even in a case where the resistance value of the termination resistor Rtt is made small, the symmetric property of the Eye pattern is lost in a driver section with a nonlinear I-V characteristic. This causes a problem in that the window width on the low side becomes narrow. The reason is clear from operating points of pull-up (P-channel transistor)/pull-down (N-channel transistor) of the driver section. FIG. 2 shows a view regarding operating points of P-channel (Pch) transistor and N-channel (Nch) transistor.
FIG. 2 shows a case where the termination voltage Vtt is made equal to the power supply voltage VDD in a driver section with a nonlinear I-V characteristic. As is apparent from FIG. 2, the operating point of the pull-up (Pch transistor) of the driver section is mainly positioned in a range lower than an ON resistance value targeted. The operating point of the pull-down (Nch transistor) is mainly positioned in a range higher than the ON resistance value targeted. This causes the symmetric property of the Eye pattern to disappear.
In order to solve the problem described so far, a driver section having a linear I-V characteristic with an ON resistance value allowing the ‘margin between the low level and the reference voltage VRef’ to become equal to the ‘margin between the high level and the reference voltage VRef’ in consideration of the resistance value of the termination resistor Rtt and the level of the reference voltage VRef is needed.
As another problem, the resistance value of the termination resistor Rtt changes according to a condition of connection between the driver section and the receiver in order to reduce an influence of reflection. In addition, the level of the reference voltage VRef also changes with the design of the receiver. For this reason, the target ON resistance value of the driver section suitable for them also changes. Therefore, it becomes necessary to be able to adjust the target ON resistance value of the driver section.
There is a known semiconductor device which is related with the invention (for example, refer to Japanese Unexamined Patent Application, First Publication, No. 2003-69413). In this semiconductor device, the output impedance of a driver section is equal to the characteristic impedance of a signal line in a system in which there is no termination resistance element on the receiver side. Thus, a technique of determining the ON resistance Rs of a transistor and realizing the linearity is disclosed.
However, the above reference is silent about a semiconductor device whose termination resistance element is present on the receiver side. Particularly, the termination resistance element is specialized in a system depending on the VDD. That is, the known technique does not disclose any semiconductor device whose output impedance of a driver section does not need to be made to match the characteristic impedance of a signal line.
As described above, in the driver section of the semiconductor device, there is needed a driver section having a linear I-V characteristic with an ON resistance value allowing the ‘margin between the low level and the reference voltage VRef’ to become equal to the ‘margin between the high level and the reference voltage VRef’ in consideration of the resistance value of the termination resistor Rtt and the level of the reference voltage VRef.
As another problem, the resistance value of the termination resistor Rtt changes according to a condition of connection between the driver section and the receiver in order to reduce an influence of reflection, and the level of the reference voltage VRef also changes with design of the receiver. The target ON resistance value of the driver section suitable for them also changes. Therefore, it becomes necessary to be able to adjust the target ON resistance value of the driver section.